As pulp mill plants strive to further decrease usage of fresh water from current levels by means of closed looped systems, the concentration of non-process elements within the manufacturing process, i.e. undesirable inorganic elements, will tend to increase in different process streams. This might have a negative impact on process chemistry, scale formation, process efficiency and product properties. Such undesirable inorganic elements are e.g. aluminum, calcium, barium, phosphorus, chlorine, potassium, manganese, cadmium, magnesium, iron, and zinc. Undesirable inorganic elements from the wood raw material and from chemicals used to manufacture bleached pulp from wood will accumulate in different process streams, due to recirculation of process streams such as bleach plant filtrates.
Accumulation of undesirable inorganic elements in a pulp mill often causes problems with scaling which leads to plugging of pipes, screens, instruments etc, which in turn may result in lost production or other process disturbances. Some elements such as iron and/or manganese may interfere with chemical reactions in bleaching and cause inferior pulp quality and decomposition of valuable hydrogen peroxide. Chloride and potassium ions accumulate in the recovery boiler system and contribute to the plugging and corrosion of the boiler tubes that may lead to lost production.
Among the different measures to minimize or eliminate the effluent from chemical pulp mills, closure of the bleach plant poses a great challenge. Therefore, management of undesirable inorganic elements would be a key to successful recovery and recycle of bleach plant effluents. Any pulp mill aiming at advanced closure must anticipate, and strive to prevent, the negative impacts of built-up undesirable inorganic elements.
A number of different treatment methods for removal of undesirable inorganic elements from wood material prior to chemical pulping have been proposed previously. Many of these methods involve changes in the water balance of the pulping process, mainly due to increased wood moisture content resulting from the treatment method.
For example, one previously proposed method for removal of metals in pulping processes is disclosed in SE 502,667. A method is described, in which comminuted fiber material is treated prior to digestion in the presence of a liquid containing a sequestering agent that forms complexes together with metals in the fiber material. The treatment is carried out prior to or during the performed pre-impregnation before digestion, and is performed at a pH value above 5.0. At least a part of the liquid containing the sequestering agent consists of spent liquor, fresh digestion liquid, effluent from bleaching processes, condensation, mains water or lake water, or mixtures thereof. The spent liquor used is suitably the spent liquor having reduced, low content of metals that is obtained at the digestion, which follows the disclosed treatment with sequestering agent.
Another method involving removal of metal ions is disclosed in U.S. Pat. No. 4,826,568, wherein a process for delignification of lignocellulosic substances is described. This process comprises treatment of a lignocellulosic material in a first stage with an inorganic acid at a pH of 1-4, in presence of diethylenetriaminepentaacetic acid or a metal salt thereof, to sequester metal ions. Then the cellulosic material is treated in a second stage with hydrogen peroxide to accelerate delignification of the lignocellulosic material and retard carbohydrate degradation during subsequent digestion. Water washing of the treated lignocellulosic material of the second stage, and digestion of the water washed lignocellulosic material in a kraft digester produce a paper pulp.
EP 0 921 228 A2 discloses a method of preparing chemical pulp from wood chips, in which method wood chips are treated in a precleaning stage prior to cooking, in order to remove process detrimental components. In the precleaning stage, wood chips are treated with e.g. bleach filtrate or evaporation condensate at a pH of 2.5-5.
However, treatment of wood chips with a water solution prior to digestion leads to increased moisture content in the chips entering the digester. The increased moisture content in the chips will result in larger volumes of spent cooking liquor. Since evaporation of spent cooking liquor consumes valuable steam, such a water treatment has a negative influence on the energy balance of the pulping process in comparison with traditional pulping methods. For economic reasons it is of great importance to minimize the created increased moisture content of the treated chips prior to cooking.
None of the prior art methods addresses the problem of created increased moisture content in the treated wood chips after reduction of the concentration of undesirable inorganic elements in the chips prior to cooking.
In pulping processes steaming is generally performed to replace air in the wood chips by steam. This may be done prior to leaching with a water solution to reduce the content of undesirable inorganic elements in wood chips, as well as prior to cooking in a digester. During the subsequent impregnation of the wood chips with leaching or cooking liquor, the steam in the chips is condensed, which leads to a lowered pressure inside the chips. This, in combination with the impregnation being performed under an elevated pressure, leads in turn to that leaching or cooking liquor penetrates into the chips. During the impregnation process, the density of the chips increases since a considerable part of the originally air-filled spaces in the chips become filled with leaching or cooking liquor, the more the higher the surrounding pressure. It is important that the density of the impregnated chips exceeds the density of the cooking liquor in a continuous digester so that the chips will sink in the digester. As the steaming treatment consumes a relatively large amount of steam, an elimination or reduction of the need of steaming treatment would involve a great energy saving to the pulp mill.